Prostate cancer is the most common malignancy in men in European and the US. Its incidence is in the first place among all male malignancies in the US, and its mortality is only second to lung cancer. In recent years, its incidence is on the rise every year in China. In addition, the histological malignancy of prostate cancer in the Chinese patients is higher than in the patients of the US. According to the survey on the survival of patients with urinary tract cancer in Shanghai, China, 80-90% of the prostate cancer patients in China are advanced prostate cancer when diagnosed and their 5-year survival rate is lower than 30%. Considering the large population of China and the number of patients with prostate cancer is surging, it is necessary to focus on the prevention and better treatment of prostate cancer. Traditional prostate cancer treatment methods include surgery, endocrine therapy, chemotherapy, and radiation therapy, and the outcome still remains to be improved. The recurrence rate after surgery is relatively high. For recurrent prostate cancer, the endocrine therapy of androgen deprivation is often necessary. The endocrine therapy of prostate cancer can be divided into three categories: castration therapy, anti-androgen therapy, and a combination of both. After the endocrine therapy continues 2-5 years, prostate cancer will often develop to become androgen-independent. For androgen-independent patients with prostate cancer, the current therapies include chemotherapy, radiotherapy, radionuclide irradiation and bisphosphonate therapy, etc, all of which are of limited effect. The treatment of prostate cancer needs improvement and it is urgent to seek a more effective therapeutic regimen. Thus, scientists and clinical experts are actively exploring relatively safe and effective therapeutic regimens. With the advances in bacterial- and viral-based gene therapy and genetic engineering technology, mounting studies have focused on bacterial treatment of tumors. Since mid-1990s, studies have found that attenuated Salmonella typhimurium can kill the tumor cells effectively in mice.
Salmonella is a group of Gram-negative, invasive intracellular facultative anaerobes parasitized in human and animal intestinal tracts. VNP20009 is an attenuated Salmonella typhimurium strain with deletion of msbB and pur I genes. It is genetically stable and sensitive to antibiotics. The msbB protein is necessary for the lipid acylation to endotoxin, and the lipid acylation at A-terminal cannot be achieved when deletion, lowering the toxicity. The pur I protein is involved in purine metabolism, deletion of this gene leads to dependence of exogenous adenine when culturing the bacteria. These gene manipulations of VNP20009 also lower the production of tumor necrosis factor (TNF), thereby reducing inflammatory response. Consequently, the low pathogenicity improves the safety of its clinical usage. VNP20009 has been widely used in cancer research, which can influence the growth of a variety of solid tumor models of mice, including melanoma, lung cancer, colon cancer, breast cancer, and renal cancer. VNP20009, as a vector of gene therapy, has the ability to accumulate in the tumor site in a highly targeted fashion. Researchers have found in the mouse models carrying a variety of solid tumors that the quantity of VNP20009 in tumors is 200-1000 times as high as that in non-cancerous major organs, such as the liver. VNP20009 can preferentially accumulate and multiply under the hypoxic and necrotic conditions in the tumor tissue. At the same time, the bacterial multiplies significantly faster in the tumor tissues than in the normal tissues, making it possible for the attenuated Salmonella to be a new type of anti-tumor agent and the vector of targeted gene therapy. Potential mechanisms for the effect of a slow tumor growth by VNP20009 may include the follows: 1) Breakdown of nutrients necessary for tumor growth by the bacteria (e.g., through the enzymes produced by bacteria such as asparaginase) can deplete essential amino acids for tumor growth; 2) Stimulation of local toxin secretion or tumor necrosis factor α to tumor microenvironment can negatively influence the tumor angiogenesis. In addition, the non-specific inflammatory reaction at the bacterial growth site can activate anti-tumor T cells. However, so far, the antitumor activity of VNP20009 on prostate cancer cells has not been revealed.
In order to maintain its high rate of reproduction, tumor cells require adequate nutrition. In addition to carbohydrates, the need for methionine (Met), glutamine, and arginine is particularly high. Previous studies have established that Met-dependency is a common feature of most tumor cells, such as breast cancer, prostate cancer, lung cancer, colon cancer, kidney cancer, bladder cancer, melanoma, glioma, etc. High Met-dependency does not exist in normal cells. Both in vivo and in vitro experiments have confirmed that dietary intervention with methionine deficiency can delay the proliferation of tumor cells. However, long-term deficiency of Met can cause malnutrition, metabolic disorders, and aggravate tumor growth due to a long-term DNA hypomethylation. Thus, by specifically degrading Met through L-methioninase and lowering the level of methionine in vivo, we will be able to effectively inhibit the growth of tumor cells or even degrade them. Experiments in animal models have confirmed that intraperitoneal injection of methioninase can inhibit the growth of Yoshida sarcoma and lung tumor in nude mice. In previous clinical trials, four patients with breast cancer, lung cancer, kidney cancer and lymphoma received methioninase injection once every 24 h. Methioninase could significantly reduce the methionine content in plasma. However, since methioninase is not natively expressed in mammalians, exogenous administration has strong side effects often related to immunological response.